1. Field of the Invention
The present invention relates to an image forming apparatus having a sheet feeding portion.
2. Description of the Related Art
Recently, there is an increasing demand for an image forming apparatus such as a copying machine or printer to copy or print highly smooth sheets such as art paper and coated paper used for flyers and posters. Unlike high-quality paper or plain paper, however, highly smooth sheets stick to each other due to moisture absorption. When the image forming apparatus is used in a humid environment, a sheet bundle in the sheet cassette absorbs moisture from the atmosphere and sticks. Reliably separating sheets one by one is difficult for the friction separation technique such as a separating pad technique or a retard separation technique.
Some apparatus uses a sheet-like moisture-proof member on the top surface of a sheet bundle stored in the sheet cassette in order to decrease moisture absorption into sheets and prevent sheets from sticking to each other. For example, the apparatus discussed in Japanese Patent Laid-Open No. 3-008637 includes a moisture-proof apparatus. The moisture-proof apparatus uses a moisture-proof sheet whose width almost equals the sheet width. The moisture-proof sheet abuts the internal surface of a width regulation plate and is regulated in terms of the lateral position. The moisture-proof sheet is placed so as to be in close contact with the top surface of sheets stored in the sheet cassette or stacked on a tray.
The sheet feeding apparatus proposed in Japanese Patent Laid-Open No. 2004-161456 separates sheets that absorb moisture and stick to each other. The sheets can be separated one by one for feeding. The sheet feeding apparatus provides a dehumidification portion on the top of a sheet bundle in the sheet cassette. The dehumidification portion includes two OHP sheets as synthetic resin plates that sandwich a sheet-like heater as a heat generator made of an SUS plate. The heater heats and dehumidifies moistened sheets so that the sheets can be separated.
To maximize the moisture-proof effect, the sheet-like moisture-proof member or the sheet-like moisture absorption portion including the heater must cover as wide a sheet surface area as possible while avoiding a sheet position regulating plate or a feeding roller around the stored sheet bundle. Differently sized moisture-proof members need to be used depending on sheet sizes. However, a user may inadvertently place a sheet bundle on the moisture-proof member or forget to set the moisture-proof member while replacing the moisture-proof member or supplying sheets to the sheet cassette. As a result, the moisture-proof member unsuccessfully provides the moisture-proof effect. Unnecessary sheets are fed.
The present invention provides an image forming apparatus capable of reliably stacking a moisture-proof member when an absorbent sheet is used for image formation.